One in a row about it recalled comparison (5)
I believe the answer is:
ratio
'comparison' is the definition.
(a ratio compares two values)
'one in a row about it recalled' is the wordplay.
'one in' means one lot of letters goes inside another.
'a row' becomes 'oar' (I've seen this before**).
'about' says the letters should be written in reverse.
'recalled' shows that the letters should be reversed in order.
'oar' reversed gives 'rao'.
'it' reversed gives 'ti'.
'rao' enclosing 'ti' is 'RATIO'.
(Other definitions for ratio that I've seen before include "Eg, 4 to 3" , "Relation between two amounts" , "figures in relationship" , "Comparative figure" , "Proporation, quantitative relation" .)